Generally, in a radio communication system employing time division duplex (TDD) in which transmission and reception is performed in a time division manner, signals of the same frequency band are transmitted and received in uplink (hereinafter referred to as “UL”) and downlink (hereinafter referred to as “DL”). Therefore, due to the symmetry of radio channels, UL and DL radio channels can be regarded as being equivalent. Using this property, for example, a base station device carries out UL channel estimation using a UL reference signal transmitted from a user terminal, regards a UL channel estimation value as a DL channel estimation value, and performs precoding of a DL signal or the like.
Meanwhile, the UL and DL radio channels are symmetrical and equivalent, while transmission characteristics of transmission and reception circuit of devices such as the base station device and the user terminal are different. For this reason, for example, the UL channel estimation value that the base station device estimates from the UL reference signal does not accurately indicate an actual DL channel.
In other words, for example, a UL signal passes through a transmission circuit of the user terminal, passes through a UL radio channel, and passes through a reception circuit of the base station device. On the other hand, a DL signal passes through a transmission circuit of the base station device, passes through a DL radio channel, and passes through a reception circuit of the user terminal. For this reason, the UL channel for which the base station device performs channel estimation includes the transmission circuit of the user terminal and the reception circuit of the base station device, while an actual DL channel includes the transmission circuit of the base station device and the reception circuit of the user terminal. Since the transmission characteristics of the transmission circuit and the reception circuit are different between the base station device and the user terminal, the UL and DL channels differ from each other due to a difference in the transmission characteristics of the transmission and reception circuits.
In this regard, there are cases in which calibration is executed in order to obtain the accurate DL channel estimation value from the UL channel estimation value. The calibration is a process of calibrating the difference in the transmission characteristic of a plurality of transmission and reception circuits, and for example, a relative relation such as phase rotation and amplitude variation in a plurality of channels is obtained by performing transmission and reception of test signals. Specifically, for example, calibration of a device 1 including a transmission circuit T1 and a reception circuit R1 and a device k including a transmission circuit Tk and a reception circuit Rk as illustrated in FIG. 13 will be described.
As transmission and reception of the test signals are performed between two antennas illustrated in FIG. 13, a channel estimation value h1,k of a channel including the transmission circuit T1 indicated by a transfer function T1, a radio channel, and the reception circuit Rk indicated by a transfer function Rk and a channel estimation value hk,1 of a channel including the transmission circuit Tk indicated by a transfer function Tk, a radio channel, and the reception circuit R1 indicated by a transfer function R1 are obtained. A ratio of the channel estimation values h1,k and hk,1 is indicated by the following Formula (1).
                                          h                          1              ,              k                                            h                          k              ,              1                                      ⁢                                  ⁢                                            R              k                        ⁢                                                  ⁢                          g                              1                ,                k                                      ⁢                                                  ⁢                          T              1                                                          R              1                        ⁢                                                  ⁢                          g                              k                ,                1                                      ⁢                                                  ⁢                          T              k                                      ⁢                                  ⁢                                            R              k                        ⁢                          /                        ⁢                          T              k                                                          R              1                        ⁢                          /                        ⁢                          T              1                                                          (        1        )            
Here, in Formula (1), g1,k and gk,1 are each a propagation path value of the radio channel and can be regarded as the same value in the transmission and reception in both directions. If a correction coefficient u1 related to the transmission circuit T1 and the reception circuit R1 of the device 1 is defined as 1 from Formula (1), a correction coefficient uk for the transmission circuit Tk and the reception circuit Rk of the device k is given as in the following Formula (2).
                              u          k                ⁢                                  ⁢                              u            k                                u            1                          ⁢                                  ⁢                                            R              k                        ⁢                          /                        ⁢                          T              k                                                          R              1                        ⁢                          /                        ⁢                          T              1                                      ⁢                                  ⁢                              h                          1              ,              k                                            h                          k              ,              1                                                          (        2        )            
As the correction coefficient uk of the device k is obtained as described above, for example, if the channel estimation value hk,1 of the signal transmitted from the device k to the device 1 is multiplied by the correction coefficient uk, an accurate channel estimation value h1,k of a channel directed from the device 1 to the device k is obtained.
For example, the calibration is executed in coordinated multi-point transmission (CoMP) in which a plurality of transmission points (hereinafter referred to as “TPs”) connected to one baseband processing device cooperate to transmit a signal to the user terminal. In a case in which signals are transmitted from a plurality of TPs, the calibration for calibrating the difference in the transmission characteristic of the transmission and reception circuits of each TP is executed. In other words, for example, the calibration of the two TPs is executed such that the test signals are transmitted and received between two TPs which are a calibration target, and the channel estimation value between the two TPs is obtained using the test signals. As described above, it is possible to implement TP calibration of the whole radio communication system by forming pairs from a plurality of TPs and sequentially calibrating each pair.    Patent Document 1: Japanese National Publication of International Patent Application No. 2013-542672    Patent Document 2: International Publication Pamphlet No. WO 2013/084991    Patent Document 3: Japanese National Publication of International Patent Application No. 2015-510282    Patent Document 4: Japanese Laid-open Patent Publication No. 2011-15318    Non-Patent Document 1: Daisuke Jitsukawa, Takashi Seyama, Takaharu Kobayashi, Teppei Oyama, Takashi Dateki, Hiroyuki Seki, Morihiko Minowa, “Study of Inter-Transmission Point Calibration Scheme for 5G Ultra High-Density Distributed Antenna Systems”, IEICE-RCS2016-142, The Institute of Electronics, Information and Communication Engineers (IEICE), August 2016
However, there is a problem that the accuracy of calibration may decrease when the calibration is performed on a plurality of TPs. Specifically, for example, in a case in which the calibration is performed such that the two TPs serving as the calibration target transmit and receive the test signals as described above, transmission and reception timings of the test signals in the two TPs may coincide with each other. If the transmission and reception timings of the test signals in the two TPs coincide with each other, the test signal transmitted from one TP interferes with the test signal received from the other TP, the accuracy of the channel estimation between the two TPs decreases, and reliability of the calibration is lowered accordingly. Meanwhile, the transmission and reception timings of the test signals in the two TPs may be significantly apart from each other. In a case in which the transmission and reception timings of the test signals in the two TPs are significantly apart from each other, a temporal variation occurs in the radio channel between the two TPs, the accuracy of the channel estimation decreases, and the reliability of calibration decreases accordingly.
In order to suppress such a decrease in the accuracy of calibration, the transmission and reception of the test signals may be sequentially performed in a time division manner for each of pairs formed from all the TPs connected to the baseband processing device. However, in a case in which the transmission and reception of the test signal are sequentially performed in a time-division manner, it takes time to complete the transmission and reception of the test signals in all the pairs of TPs, and radio resources (particularly, time resources) are be wasted.